Justin Volpe — the cop who tortured Abner Louima — told a lawyer for another accused cop he’d “say nothin'” about that officer’s alleged role in the brutal attack when he pleaded guilty to the assault, prosecutors said yesterday.

In an explosive letter to the judge in the federal conspiracy trial spawned by the attack on Louima, prosecutors said they want to call witnesses to testify they overheard the conversation between Volpe and Stephen Worth, former lawyer for ex-cop Charles Schwarz.

The letter, which was revealed yesterday, claims the talk took place on May 20, 1999, at the Brooklyn federal courthouse during a lunch break in the first trial — while Volpe’s lawyer, Marvyn Kornberg, was on the fifth floor trying to negotiate a plea deal with prosecutors.

But in the letter, prosecutors charge that proves Schwarz — who was convicted last year of holding Louima down while Volpe sodomized him in the 70th Precinct station-house bathroom — is guilty of the crime.

Defense lawyers say Schwarz didn’t participate in the assault, and plan to call Volpe to the stand to testify to it.

Worth, in Albany where he’s representing a cop in the Amadou Diallo police-shooting trial, refused to comment on the letter, saying only, “They know where to find me. I’d be happy to respond to a subpoena.”

Ronald Fischetti, who now represents Schwarz, said he hadn’t gone over the letter and couldn’t comment.

But the move could force defense lawyers to call Worth to testify.

Judge Eugene Nickerson will rule on the letter next week.

Schwarz and Officers Thomas Bruder and Thomas Wiese are on trial for allegedly covering up Schwarz’s role in the Louima attack.

Volpe pleaded guilty to sodomy on May 25, 1998, and said a second officer was in the bathroom with him. He’s since said it was Wiese, not Schwarz.

Kornberg said he didn’t hear the alleged Worth-Volpe conversation, and wouldn’t comment — but said the government is “grabbing at straws.”

Also yesterday, Officer Mark Schofield acknowledged in cross-examination that he told a series of investigators he didn’t know who walked Louima to the bathroom before the attack — although he had told a federal grand jury it was Schwarz.